In Thailand the timetable is the climate, and the rest is a hub city plus one other region if you want the week to remain pleasant. Bangkok is hot in every month you are likely to come. Walk early. The skytrain, river boats and taxis work; the heat does not care. The north around Chiang Mai is a different temperature in the hills. The gulf islands and the Andaman coast have monsoons that peak in different halves of the year, so the “best beach” depends on the month, not on a ranking.
Between Bangkok and the rest of the country you fly, or you take a night train, or you accept a long bus. Between islands the product is often a ticket that is a minivan plus a ferry plus another minivan, sold from a booth or an app. Those legs used to be arranged entirely by touts at the pier. They still can be. The less chaotic version is to book the boat and the airport transfer before you are standing in the sun with a suitcase.
Klook is widely used in Thailand for exactly those pieces: island ferries, airport cars, day tickets, the things that are real activities rather than souvenirs. It is a middleman. It is also how you avoid discovering at 06:00 that the only boat today left at 05:30. You can still buy a mango at street price. You cannot always walk onto a speedboat to Koh Tao at the hour you invented.
Thai is the language; a little of it changes how people treat you. English works in the tourist layer. The baht is cash for street food and small islands, cards for malls and hotels. Temples want covered shoulders and knees. Bangkok plus the north, or Bangkok plus one coast, is a full first trip. Both coasts, a hill trek and a cooking class in nine days is a blur of vans.
